Having had a quick look at a superscript manual, it seems that you should be able to get cDNA up to 15 kb in length. Perhaps you are having some issues with where the random hexamers are binding. Could you try oligo dTs or specific primers (if the sequence is known)? You may have better luck with that. You may also want to try a longer incubation time to allow time for the longer product to synthesise fully.

You may also need to play with the ratio of hexamer to RNA to get an optimal yield for longer products, but I'm just guessing here.